Emissions such as flue gases from fossil fuel from combustion facilities of coal-fired utilities and municipal solid waste incinerators typically include sulfur trioxide (i.e., SO3). Coal and petroleum fuels usually contain sulfur compounds. Combusting coal and petroleum typically results in generation of sulfur dioxide (i.e., SO2).
Combustion facilities typically operate pollution control devices to remove nitrogen monoxide and nitrogen dioxide (NOx). A side effect of operating NOx control devices (Selective Catalytic Reduction SCR) is the generation of sulfur trioxide. In the SCR, a portion of the sulfur dioxide emission is typically oxidized, resulting in sulfur trioxide. Sulfur trioxide has a strong affinity for water, and, in the presence of moisture and low temperatures, easily converts into sulfuric acid (H2SO4). In ambient conditions, typically all SO3 is converted into H2SO4, such that there is almost no presence of SO3 in air.
Sulfur trioxide emissions are undesirable for several reasons. As briefly mentioned, sulfur trioxide and/or sulfuric acid exiting a stack or chimney can result in pollution such as acid rain. Sulfur trioxide can be very corrosive to equipment used in combustion facilities.
Selective catalytic reduction processes used to reduce pollutants in a flue gas can have the effect of creating higher levels of sulfur trioxide in a flue gas. Such higher levels of sulfur trioxide have been known to adversely affect removal of certain pollutants. For example, mercury is commonly removed from flue gases using activated carbon as part of an adsorption process. With higher levels of sulfur trioxide present during the adsorption process, the percent of mercury removed from a flue gas typically decreases.
There are several types of conventional detection systems that can be used to detect a presence of sulfur trioxide. Such systems include Fourier transform infrared (FTIR) spectroscopy, tunable diode laser spectroscopy (TDL), acid dew-point, conversion-fluorescence, filter-correlation, and cavity ring-down spectroscopy.
There have been attempts to use FTIR and tunable diode laser spectroscopy to measure sulfur trioxide. Compared with TDL, FTIR showed less sensitivity and might not be feasible for monitoring sulfur trioxide in stack gas. Acid dew-point and conversion-fluorescence methods could not distinguish between sulfuric acid and sulfur trioxide and also suffer severe interferences. Like FTIR, filter-correlation may not be sensitive enough for measuring sulfur trioxide as continuous environmental monitoring system (CEMS). Cavity ring-down spectroscopy is a very sensitive and highly selective technology for gas sensing. It relies on very high minor reflectivity (larger than 99.99%) for its up to 104 optical passes. The highly contaminated gas samples might quickly deteriorate the minor reflectivity and potentially limit its application to the stack gas monitoring. Accordingly, such techniques may not be useful for detecting SO3.
By and large, most conventional systems measure an amount of sulfuric acid in a sample and then use such a measurement to infer an amount of sulfur trioxide in a gas sample. Measuring sulfuric acid is so common that many of today's systems claim to measure an amount of sulfur trioxide, yet in reality, such systems actually measure an amount of sulfuric acid in a sample to infer an amount of sulfur trioxide. In other words, many “SO3” measurements in the prior art are inferred through surrogate measurements of H2SO4.